Methods for preventing oxygen inhibition of a light-initiated polymerization reaction in a 3d printing system using inert gas

ABSTRACT

Methods that prevent oxygen inhibition of a light-initiated polymerization reaction by purging the oxygen from reaction surfaces using inert gas flow. In some embodiments, oxygen is purged using a gas diffusion system that introduces, via a diffuser, an inert gas into a workspace between a UV light source and a UV curable layer of a workpiece. The diffuser may be made of a transparent or diffuse material to allow UV light to pass through it, and includes an array of micro-holes for the gas to pass through towards the workpiece. The inert gas flow may be heated to maintain a desired and uniform reaction temperature.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/703,417, filed on 4 Dec. 2019, which is a NONPROVISIONAL of, andclaims priority to U.S. Provisional Application No. 62/777,902, filed 11Dec. 2018, both of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a system that prevents oxygeninhibition of a light-initiated polymerization reaction used by a 3Dprinting system by purging the oxygen from the reaction surface usinginert gas flow.

BACKGROUND

Many additive manufacturing, or so-called three-dimensional (“3D”)printing, applications use ultraviolet (“UV”) light-curable polymers.The UV curing process consists of three stages: photoinitiation,propagation, and termination. During photoinitiation, a photoinitiatorproduces free radicals when exposed to UV radiation. These free radicalsreact with nearby monomers and convert them into free radicals. Next, inthe propagation stage, the free radical monomers bond with othermonomers and turn those monomers into free radicals. In this way themonomers form a polymer chain. The process continues until it reachestermination. Termination can occur in many ways, including if two chainsbond with one another, the free radical transfers to a monomer, or ifthe chain reacts with molecules from the environment and not a monomer.

There are two interactions between oxygen and the photopolymer thatinhibit curing: quenching and scavenging. After the photoinitiator hasbeen excited by exposure to UV radiation, it produces a free radical.Molecular oxygen easily reacts with this free radical, preventing itfrom reacting with monomers in the process of chain propagation. This isthe quenching reaction. This reaction also produces an oxygen freeradical. In the scavenging reaction, this oxygen free radical reactswith a free radical that is part of a propagating polymer chain. Thisreaction results in a less reactive free radical, which leads to earlytermination of the polymerization process. These two processes can bewritten as:

PI*+O₂→PI+O₂*  Quenching reaction:

R.+O₂*→R—O—O.  Scavenging reaction:

Because of these phenomena, if a photopolymer is exposed to oxygenduring curing in a 3D printing process it can result in uncured polymerresidue on surfaces exposed to the air.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a UV curing system includes a gasdiffusion system for introducing an inert gas into a workspace between aUV light source and a UV curable layer of a workpiece. A transparentcover separates the UV light source and the workspace and the inert gas(e.g., Ar, CO₂, He, Ne, etc.) flows in from gas inlets and out through adiffuser towards the workpiece. A gas pressure homogenizer is used toensure constant pressure throughout the system.

The diffuser is made of a transparent or diffuse material to allow UVlight from the UV light source to pass through it. The diffuser includesan array of micro-holes for the inert gas to pass through towards theworkpiece. The small diameter of the holes allows a closed-packed arraythereof so that the gas is evenly distributed throughout the workspace(i.e., throughout the curing area). The small diameter of the holes alsomeans that a larger area of the surface of the diffuser is free of holesmaking its optical properties more homogenous. This ensures a relativelyeven light distribution. The holes are covered with “bridges” of theUV-transparent material of which the diffuser is made. This ensures thatall light passing through the diffuser passes through at least somethickness of the transparent material, further improving lightdistribution.

After the UV curable material has been deposited on the surface of theworkpiece, and the workpiece introduced into the workspace of the UVcuring system, the inert gas is pumped through the diffuser. This flowof gas purges the oxygen from the region of the workspace adjacent tothe diffuser. The thickness of this region is related to the gaspressure as it is forced through the diffuser. With the workpiecemaintained in the area of the workspace from which oxygen has beenpurged, the UV curing system then cures the layer of UV curable materialthrough exposure to light from the UV light source.

A further embodiment of the invention provides for preventing oxygeninhibition of a light-initiated polymerization reaction by periodicallyemitting a UV light from a UV light source into a UV curing space inwhich a workpiece having a layer of UV curable material is disposed tofacilitate, within the UV curing space, UV curing of the UV curablematerial, and purging oxygen from the UV curing space at times when theUV light source emits light onto the layer of UV curable material.Purging oxygen from the UV curing space includes introducing, via a gasdiffusion system, an inert gas into a workspace between the UV lightsource and the layer of UV material of the workpiece. For example, theinert gas may introduced via one or more gas inlets of the gas diffusionsystem and through a plurality of micro-holes in a transparent diffuserseparating the UV light source and the workspace towards the workspace.The UV light from the UV light source may be transmitted through bridgesof a UV transparent material arranged over the micro-holes of saiddiffuser towards the layer of UV material of the workpiece. Thus, theinert gas and the UV light are each approximately evenly distributedthroughout the workspace via the micro-holes.

In some embodiments of the invention, the inert gas flow is used toevenly heat the UV curable material during curing or to control thetemperature of the UV curing space by controlling the inert gastemperature.

These and further embodiments of the invention are described below withreference to the accompanying drawings, in which the present inventionis illustrated by way of example, and not limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c illustrate points in a conventional 3D printing process inwhich an object to be printed (FIG. 1a ) has deposited thereon a layerof UV curable material (FIG. 1b ), which is subsequently cured throughexposure to UV light (FIG. 1c ).

FIG. 2 illustrates a UV curing system configured in accordance with oneembodiment of the invention in which an inert gas flow is arranged toprevent oxygen inhibition of the polymerization process during a UVcuring process.

FIGS. 3a and 3b illustrate aspects of the operation of the UV curingsystem shown in FIG. 2.

FIGS. 4a and 4b illustrate an example of a UV light source and gasdiffuser arrangement for the UV curing system shown in FIG. 2.

FIGS. 5a and 5b illustrate aspects of the gas diffuser arrangement shownin FIG. 4.

FIGS. 6a and 6b illustrate aspects of the operation of the UV curingsystem shown in FIG. 2, in particular a sequence of printing, inert gasflow, and UV curing processes (FIG. 6a ), and the expanse of the oxygenfree layer (FIG. 6b ).

DETAILED DESCRIPTION

Before describing the invention in detail, it is helpful to present anoverview. Referring to the sequence of images shown in FIGS. 1a, 1b, and1c , in many 3D printing processes in which an object 10 is undergoingfabrication, a materials printing system 12 is used to deposit UVcurable material 14 on a surface 16. This deposited material is thencured with a UV light source 18 to produce a new layer of the desiredpart 10′. This process continues until the part undergoing fabricationis completed.

Embodiments of the invention provide systems and methods for preventingoxygen inhibition of a light-initiated polymerization reaction atambient conditions. Referring now to FIG. 2, in one embodiment of thepresent invention a UV curing system 20 is equipped with a gas diffusionsystem 22. A transparent cover 24 is disposed between the UV lightsource 26 and the gas diffusion system 22. The gas flows in from gasinlets 28 and out through a diffuser 30 at the bottom of the system. Agas pressure homogenizer 32 is used to ensure constant pressurethroughout the system.

The diffuser 30 is made of a transparent or diffuse material to allow UVlight to pass through it onto a workpiece 34, and in particular onto alayer of UV curable material 36 disposed thereon. The diffuser 30consists of an array of micro-holes 38. The small diameter of the microholes allows for a closed-packed array thereof so that the gas is evenlydistributed throughout the curing area 40. The small diameter of themicro-holes 38 also means that a larger area of the surface of thediffuser 30 is free of holes, making its optical properties morehomogenous. This ensures more even light distribution. Of course, otherarrangements and sizing of the micro-holes may be employed so as tooptimize gas distribution and light distribution throughout the curingarea. The micro-holes 38 are covered with “bridges” 42 of the materialof which the diffuser is made. This ensures that all light passingthrough the diffuser must pass through some region of the transparentmaterial. This further improves the light distribution.

Referring now to FIG. 3a , after the UV curable material 36 has beendeposited on the print surface, the gas is pumped through the diffuser30 via the gas inlets 28. This flow of gas purges the oxygen from aregion 44 adjacent to the diffuser 30. The thickness of this region isrelated to the gas pressure as it is forced through the diffuser.Thereafter, as shown in FIG. 3(b), if necessary the device underfabrication is raised so that the layer of UV curable material 36disposed thereon is disposed within the oxygen-free region 44, and theUV source 26 of the UV curing system 20 is activated, thereby curing atleast a portion 36′ of the layer of UV curable material 36 of the partundergoing fabrication in a region exposed to the UV light 48. In someinstances, it will be unnecessary to move the device under fabricationbecause the layer of UV curable material 36 will already be within theoxygen-free region 44 when the gas is pumped through diffuser 30. Inembodiments of the invention, the gas pumped through diffuser 30 ispreferably an inert gas (e.g., Ar, CO₂, He, Ne, etc.) insofar as it doesnot interact with the photopolymer in UV curable layer 36 so as toinhibit curing thereof.

In some embodiments, the temperature of the feed gas may be controlled(e.g., through heating provided prior to gas inlets 28 and/or within thegas diffusion system 22) to create a uniform reaction temperature in thevicinity of workpiece 34 (e.g., within a space within which curing ofthe layer of UV curable material 36 disposed on the surface of theworkpiece 34 will take place). For example, the inert gas may be heatedprior to its introduction into the gas diffusion system 22 so as tomaintain a desired and uniform reaction temperature within the vicinityof the surface of workpiece 34 on which the layer of UV curable material36 is disposed.

FIGS. 4a and 4b illustrate one example of a gas diffusion system 22. InFIG. 4a , a front cover 50 is in place, while in FIG. 4(b) it has beenremoved to show aspects of the interior of the gas diffusion system 22.In this example, gas diffusion system 22 is a rectilinear box having aUV light source 26, for example made up of one or more light emittingdiodes (LEDs), mounted therein, inside a top of the box. A gas diffuser30 forms a bottom face of the box. As mentioned above, the gas diffuseris made of a transparent (at the wavelengths of illumination necessaryfor curing of the photocurable material used to fabricate the part underconstruction) material to allow UV light from source 26 to pass throughrelatively unattenuated.

FIGS. 5a and 5b highlight the construction of diffuser 30. As notedabove, bridges 42 (in the illustrated example, fashioned as ribs runninglongitudinally across an upper surface of the diffuser 30) are disposedabove gas flow holes 38 so that they are in an optical path between theUV light source 26 and the gas flow holes when the diffuser and UV lightsource are assembled in the gas diffusion system 22. This ensures thatacross the entire curing area the UV light will pass through at leastsome thickness of the transparent material. This ensures better lighthomogeneity and more even curing of the photopolymer in UV curable layer36.

Returning to FIG. 4b , gas enters (e.g., by action of a pumpingarrangement) the diffusion system 22 via one or more inlet holes 28 andexits through the diffuser 30. A gas pressure homogenizer (not shown inthis view) is used to ensure constant pressure throughout the system.

FIGS. 6a and 6b illustrate the cooperating operations employed in theprinting and curing process. Printing of a next layer (6 a-10) beginswith the deposition of a layer of UV curable material on the printsurface of an object under fabrication. Towards the end of thisdeposition, gas is pumped 52 through the diffuser 30 via the gas inlets28, as shown in the gas pressure curve in FIG. 6a . The gas pressureincreases to the level required for the curing process (6 a-20) andpurges the oxygen from a region 44 adjacent to the diffuser 30. Thethickness (H) of this region grows over time, as shown in the oxygenfree area thickness curve in FIG. 6a , and is related to the gaspressure as it is forced through the diffuser. When a desired thicknessH* has been attained, the device under fabrication is raised (ifnecessary) so that the layer of UV curable material disposed on theworkpiece is within the oxygen-free region 44, and the UV source 26 ofthe UV curing system 20 is then activated (6 a-30), as shown in the UVsource curve in FIG. 6a . This causes curing of at least a portion ofthe layer of UV curable material disposed on the workpiece in a regionexposed to the UV light. At the conclusion of the curing (6 a-40), theworkpiece is repositioned for deposition of the next layer of UV curablematerial and the gas pressure reduced. Preferably, the gas pressure ismaintained at a sufficient level to keep the diffusion system 22 filledin order to reduce the time necessary for the next cycle of curing. Whenthe desired number of layers have been cured, the process ends.

Thus, methods that prevent oxygen inhibition of a light-initiatedpolymerization reactions by purging the oxygen from reaction surfacesusing inert gas flow have been described.

What is claimed is:
 1. A method, comprising: depositing a layer ofcurable material on a print surface of an object under fabrication;while depositing the layer of curable material on the print surface,initiating a purging of oxygen from a region adjacent to a diffuser;upon finishing the deposition of the layer of curable material on theprint surface, transporting the layer of curable material into theoxygen free region adjacent to the diffuser while continuing to purgeoxygen from the region adjacent to the diffuser; and curing the curablematerial while continuing to purge oxygen from the region adjacent tothe diffuser.
 2. The method of claim 1, wherein curing the curablematerial comprises curing the curable material with ultra-violet (UV)light.
 3. The method of claim 2, the diffuser comprises a UV-transparentmaterial, and curing the curable material comprises shining the UV lightthrough the diffuser.
 4. The method of claim 1, wherein purging oxygenfrom the region adjacent to the diffuser comprises pumping an inert gasthrough the diffuser.
 5. The method of claim 4, wherein the inert gas isheated prior to being pumped through the diffuser so as to maintain auniform temperature within a vicinity of the curable material.
 6. Themethod of claim 4, wherein the inert gas comprises one or more of argon(Ar), carbon dioxide (CO₂), helium (He) and neon (Ne).
 7. The method ofclaim 1, wherein transporting the layer of curable material into theoxygen free region adjacent to the diffuser comprises raising the objectunder fabrication.
 8. The method of claim 1, further comprising afterfinishing the curing of the curable layer, transporting the object awayfrom the diffuser.